High average power diode pumped solid state lasers often have beam modulation or non-uniform spatial profiles in the near field and can have a Gaussian or multi-peaked far field. Several important laser applications including pumping other lasers, machining, laser shock generation, and laser ablation require a flat spatial profile either in the near field or the far field.
Video projectors utilize beam homogenizers in order to provide images with uniform intensity as a function of screen position. Typically, these beam homogenizers are lens based homogenizers. FIG. 1 is a simplified drawing of a conventional microlens array beam homogenizer. As illustrated in FIG. 1, a first two-dimensional square array of microlenses LA1 is positioned a predetermined distance d from a second two-dimensional square array of microlenses LA2. The microlens arrays are plano-convex with parabolic lens profiles. The distance d is equal to the focal length of the second two-dimensional square array of microlenses fLA2. The first two-dimensional square array of microlenses LA1 focus incoming collimated light at a distance equal to fLA1. Light from each of the microlenses in the first array is thus spread to several microlenses at corresponding adjacent positions in the second array. The output of the second two-dimensional square array of microlenses LA2 is a collimated and uniform beam that can be focused by lens L to a top hat profile in the far field, which is at the focal length fL of lens L.
The refractive microlens arrays illustrated in FIG. 1 includes features (i.e., the microlenses) that are large in comparison to the wavelength of light. As a result, structure associated with these features is present in the semi-homogenized beam. Because some embodiments of the present invention utilize diffraction structures on the order of the wavelength of light, such larger structural nonuniformity is avoided in the homogenized output beams produced using embodiments of the present invention, resulting in greater beam uniformity and a reduction in intensity variations.
FIG. 2 is a simplified perspective drawing of a conventional crossed cylindrical lens beam homogenizer. The homogenizer illustrated in FIG. 2 utilizes two condensers C1 and C2. Each condenser, for example, condenser C1 includes refractive, plano-convex, cylindrical lens arrays 210 and 212 with parabolic lens profiles. The cylindrical lenses 210 and 212 are mounted on either side of a central glass plate 214. Light is focused by the first linear array of cylindrical lenses 210 at a distance f1 as illustrated by light rays 220. Light is focused in an orthogonal dimension by the second linear array of cylindrical lenses 212 with a focal distance f2 as illustrated by light rays 222. A second condenser C2 with matching sets of cylindrical lens arrays is positioned at a focal distance from the first condenser.
Despite the availability of microlens array and crossed cylindrical lens beam homogenizers, there is a need in the art for improved methods and systems for homogenizing laser beams in high power laser applications.